Understanding Tank Water Quality

The drawing on the right shows a fairly typical rainwater tank layout.

I have seen lots of tanks set up like this and have also seen this layout in tank supplier’s brochures.

The set up is probably OK for garden watering and toilet flushing but not much else………………………..”So what are the Issues?”

Variable Water Quality From Top to Bottom

Even with ‘leaf screens’ and ‘first flush divertors’ there is going to be some particles in the water coming of your roof, These particles will either be lighter than water and float to the top, or heavier than water and sink to the bottom.

The smaller the particle the longer it will take to sink to the bottom.

The 2nd diagram shows how the water quality varies through the tank a few days after it has rained.

  • There are some particles floating on the surface.
  • There is some material close to the bottom which can include rotting organic matter. Sometimes called the Anaerobic zone.
  • The water between the bottom and the top gradually improves as the height increases with the best water being about 1 cm below the surface.


Because of the variable water quality problems are:

  • The outlet is close to the zone of worst water quality.
  • When it rains the turbulence from the inlet mixes the tank which then takes time to settle.
  • The overflow takes some of the better quality water.

Over the next few weeks I will provide  information about ways of improving the water quality in your tank.


For more about tank water quality see Rainwater Safety



  1. baiden

    My tank is like the one on the top but I only use it on the garden.

    Do I need to do anything about water quality?

    1. Brian Ashworth (Post author)

      If you are just going to use it with a hose it will probably be fine.

      If you have got an irrigation system with microsprays or drippers a good filter is a must.


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